There has been well known a vehicle brake system including a booster for boosting a depression force when a brake pedal is depressed, for example, Patent Document 1 discloses a vehicle brake system (an electric brake actuator) using an electric motor as a boost source. The vehicle brake system disclosed in Patent Document 1 includes an axis member moving back and forth through an operation of the brake pedal as a main piston, and a cylindrical member being fitted on the axis member (main piston) as a booster piston, and is configured to boost the depression force by generating a brake hydraulic pressure with a thrust inputted from the brake pedal to the axis member (main piston) and a thrust applied by the electric motor to the cylindrical member (booster piston).